[Macros] Set -external-plugin-path when the toolchain is not Xcode

Author: rintaro

Sources/SwiftDriver/Jobs/FrontendJobHelpers.swift

@@ -280,6 +280,30 @@ extension Driver {
       commandLine.appendPath(localPluginPath)
     }
 
+    // Pass -external-plugin-path if the current toolchain is not a Xcode
+    // default toolchain.
+    if
+      isFrontendArgSupported(.externalPluginPath),
+      let darwinToolchain = toolchain as? DarwinToolchain,
+      let xcodeExecutableDir = try darwinToolchain.findXcodeExecutableDir(),
+      try xcodeExecutableDir != toolchain.executableDir
+    {
+      let xcodePluginServerPath = xcodeExecutableDir
+        .appending(component: "swift-plugin-server")
+
+      let xcodePluginPath = xcodeExecutableDir
+        .parentDirectory // bin
+        .appending(components: "lib", "swift", "host", "plugins")
+
+      if
+        fileSystem.isExecutableFile(xcodePluginServerPath),
+        fileSystem.exists(xcodePluginPath)
+      {
+        commandLine.appendFlag(.externalPluginPath)
+        commandLine.appendFlag(xcodePluginPath.pathString + "#" + xcodePluginPath.pathString)
+      }
+    }
+
     // Pass down -user-module-version if we are working with a compiler that
     // supports it.
     if let ver = parsedOptions.getLastArgument(.userModuleVersion)?.asSingle,

Sources/SwiftDriver/Toolchains/DarwinToolchain.swift

@@ -421,3 +421,18 @@ private extension Version {
     return self.description
   }
 }
+
+extension DarwinToolchain {
+  func findXcodeExecutableDir() throws -> AbsolutePath? {
+    let result = try executor.checkNonZeroExit(
+      args: "xcrun", "-toolchain", "default", "-f", "swiftc",
+      environment: env
+    ).trimmingCharacters(in: .whitespacesAndNewlines)
+
+    guard !result.isEmpty else {
+      return nil
+    }
+    return try AbsolutePath(validating: result)
+      .parentDirectory // swiftc
+  }
+}